Field of the Invention
The present invention relates to a fiber laser apparatus and a method of manufacturing an amplifying coil.
Description of the Related Art
In recent fiber laser apparatuses, output light power exceeding 1 kW can be achieved by increasing luminance of a pumped semiconductor laser or commercializing the amplifying double clad fiber. Conventionally, such a high output fiber laser can be applied to the field of material processing, in which carbon dioxide laser has been mainly used. The fiber laser has better bears quality and light condensing performance than the carbon dioxide laser. For this reason, in the high output fiber laser, since processing time can be shortened, throughput is improved, and since processing characteristics equivalent to high power even at low power can be realized, energy saving is improved. In addition, since spatial optical components are unnecessary, there are also advantages that durability is provided, problems with respect to alignment of optical components or the like do not occur, and maintenance is unnecessary.
A case where a double clad fiber having a first cladding (inner cladding) and a second cladding (outer cladding) is adopted as an amplifying optical fiber is assumed.
For example, if a cross-sectional shape of the first cladding has a shape having rotational symmetry such as a regular hexagon shape, light propagating in the first cladding spirally travels while being entirely reflected at a constant angle at an interface between the first cladding and the second cladding. In this way, pumping light which is once deviated from a core travels in the first cladding without changing the incident angle (reflection angle) with respect to the interface. Therefore, this pumping light does not reach the core no matter how many times it is reflected. Accordingly, the light substantially contributing as the pumping light is only a portion of the light that is launched into the amplifying optical fiber, and there is a problem that utilization efficiency of the pumping light is low.
In order to solve this problem, Japanese Unexamined Patent Application, First Publication No, 2001-13346 (hereinafter, referred to as Patent Document 1) below discloses an optical amplifier in which a first cladding has two or more linear reflecting surfaces, and which includes a double clad fiber in which directions of the reflecting surfaces with respect to a core are different from each other at an arbitrary location in a light propagation direction.
For example, in order to obtain the double clad fiber in which the directions of the reflecting surfaces are different from each other at an arbitrary location in the light propagation direction, Patent Document 1 discloses that the double clad fiber may be fixed to a system in a twisted state. However, it may be difficult to fix an optical fiber in a twisted state to a portion of the apparatus by a fixing jig or the like, for example and to further maintain the twisted state of the optical fiber. In addition, in a pre-step in which the optical fiber is wound in a coil shape, and thereafter, the optical fiber is fixed to the apparatus, a trouble such as dispersion of the wound optical fiber may occur. Therefore, there is a problem that handling with the coil alone is difficult.